AoC 2019 D9: Sensor Boost

To add support for relative mode, the relative_base is added to the Program class. Because eval_param and write_to now relies on two states: the memory and the relative base, I just make them member functions so they can directly access on this. Some other things are also incorporated into the class.
All memory values are now long long instead of int.
The memory is now a map<long long, long long> instead of a vector<long long>, so that the memory can be indexed at arbitrary addresses without greatly growing its physical size.